1. Field of the Invention
This invention is directed to electrical contactors, and particularly microprocessor based electrical contactors in which contactor opening is controlled with regard to the ac control voltage to vary the polarity, and in multiphase contactors, to also vary the phase sequence in which current is interrupted to thereby distribute contact wear.
2. Background Information:
Magnetic contactors are electrically operated switches used for controlling motors and other types of electrical loads. An example of such an electrical contactor is disclosed in U.S. Pat. No. 4,720,763. These magnetic contactors include a set of movable electrical contacts which are brought into contact with a set of fixed contacts to close the contactor. The contacts are biased open by a spring. The movable contacts are carried by the armature of an electromagnet. Energization of the electromagnet overcomes the spring forces and closes the contacts.
When a magnet contactor is operated by a manual push button, the opening process begins at the time of push button contact separation. There is a time delay between push button contact separation and the separation of the magnet and armature which hold the contactor closed. This delay time is a function of the magnitude of the supply voltage at push button contact separation, contact coil turns, coil resistance and coil inductance. Since the magnitude of the voltage is randomly selected by the operator of the push button, and the magnitude of the coil turns and inductance are constant and the coil resistance is approximately (varies with temperature) constant, the delay varies randomly within a range determined by the range of voltage that is available. After the delay time, opening time is a function of mechanical characteristics of the contactor including friction, spring force, and mass of the moving parts. These mechanical characteristics are considered to be constant. The total opening time is the sum of the delay time and the mechanical opening time.
When a magnetic contactor is operated by a solid state device, the operation is similar to a contactor operated by a manual push button except that the voltage applied to the coil at opening is constant due to the characteristic opening of solid state devices on a particular portion of the ac sine wave. Since all the parameters involved in contactor opening are constant, time for contactor opening is also constant.
It is well known that if an ac contactor repeatedly opens at the same point on the current wave form, its contact wear will be accelerated by a condition called material transfer. A small amount of contact material is always transferred from one contact face to the other during current interruption. The direction of material transfer is determined by the direction of current at the time of interruption. If the current, at the time of interruption is always the same, contact wear will be poor and will be evidenced by the creation of a valley on one contact face and a corresponding mountain on the other contact face. This is a common condition with dc contactors. This also occurs commonly with a solid state controlled ac contactors due to synchronism between the line voltage and the switching of the solid state device.
Another factor is involved in a three-phase magnetic contactor. Interruption of current on a three-phase circuit occurs at zero crossings. The current in the first phase to go through a zero crossing after contact separation will be interrupted first. The remaining two phases then form a single phase circuit. This single phase current in the remaining two phases will be interrupted at the next zero crossing of the single phase current. The first phase to interrupt does less work due to less arcing time than the other phases. Thus, if the first phase to interrupt current is always the same, the wear on the contacts will not be even.
A typical solution to the problem of contact wear in three-phase magnetic contactors is to use a drum switch or other device having random opening in place of a solid state control.
There is a need, therefore, for a means for evenly distributing contact wear on magnetic contactors operated by solid state control.